This invention relates generally to an electronic tone-generating system for a musical instrument which simulates the tones of a conventional, non-electronic instrument and, more particularly, to such a system which is especially adapted for use in an electronic piano.
Efforts have been made heretofore to develop electronic tone-generating systems for simulating the overall tonal quality and response characteristics of conventional musical instruments, especially percussion instruments. Because of the time-varying harmonic structure and complex envelope characteristic of a percussive tone, it has been difficult or impossible, either from a technical or a financial standpoint, to generate electronically the requisite composite signal to simulate true percussive tones.
For example, each of the notes or tones of a conventional piano with stretched wires percussively actuated by a hammer (hereinafter generally referred to as "true" piano tones) inherently possesses a tonal envelope which initially reaches a maximum magnitude or intensity rather quickly and which thereafter decays at a predetermined or inherent rate until vibration stops, either naturally after an extended time period or upon release of the key which causes a damper to engage and stop the vibrating piano wire. Moreover, the initial striking of the wire stretches it which causes its resonant frequency to be slightly lower than its unstretched resonant frequency. Various studies have shown that for these and other reasons the character of a true piano tone is dependent upon the combination of over 30 inharmonic partial frequencies, and that this partial frequency structure is continually changing during the decay period of the tone in an almost random manner.
Conventional electronic tone-generating systems for simulating the tones of a piano typically generate the different tones by combining two separate signals. The first is a uniform-amplitude oscillatory signal having a fundamental frequency approximately that of the true piano tone plus some of the harmonics thereof. The second signal is typically referred to as an envelope signal and it represents the intensity with which the piano key is struck and the duration or length of time that the key is depressed. Thus, for realistically simulating a piano tone, each of these two electronically-generated signals must be relatively complex and, consequently, the electronic generation thereof has presented significant problems.